The formation of oxide layers are important steps in the manufacturing of semiconductor devices. In thermal oxidation, an oxide film is grown on a slice of silicon by maintaining the silicon in an elevated temperature in an oxidizing ambient, such as dry oxygen or water vapor. Thermally grown silicon dioxide is used to form a stable gate oxide for field effect devices, for example.
Controlling the gate oxide thickness is an important manufacturing process control issue. As the gate oxide thickness is reduced to below 150 .ANG., the growth kinetics changes from parabolic to linear with time. This is explained in Grove, The Physics and Technology of Semiconductor Devices, pages 22-33. In other words, for gate oxides, once the thickness is above 150 .ANG., it is a self-limiting process and therefore makes it easier to control the gate oxide thickness and reduce the variance between devices. However, for gate oxides of less than 150 .ANG., the linearity of the growth kinetics with time makes control over the gate oxide thickness much more difficult. The oxide thickness as a function of time may be expressed as: ##EQU1##
with
H=Henry's Law constant PA1 P.sub.G =bulk gas pressure PA1 D=Diffusivity of O.sub.2 is Si PA1 N.sub.1 =2.2.times.10.sup.22 SiO.sub.2 molecules/cm.sup.3 in the oxide PA1 k.sub.s =chemical surface-reaction rate constant for oxidation PA1 h=gas phase mass transfer coefficient
In a typical oxide/diffusion arrangement, a wafer carrier is positioned within an oxide diffusion tube, this wafer carrier holding a number of wafers on which a gate oxide layer is to be grown. The processing of the wafers in the oxide diffusion tube, usually made of quartz, involves providing a supply of gas containing the oxidizing medium, such as oxygen or water vapor, so that it flows through the oxide diffusion tube. An oxidation furnace concentrically surrounding the oxide diffusion tube is used to heat the tube. The process is normally performed at ambient atmospheric pressure.
The thickness of the oxide layer is normally controlled through varying either the temperature and/or the furnace time, i.e. the amount of time the wafers are subjected to the gas containing the oxidizing medium and the elevated temperature. Although strict control is made of the temperature and the flow of gas through the oxide diffision tube the variance in the gate oxide thickness tends to be approximately ten percent.